The data you see within a dashboard will be based on your current access within your account. For example, if you have access to folder X that contains 10 project grids, you will see data from all 10 project grids. If you have access to folder Y that contains 20 project grids, but you only have access to 10 of those grids, you will only see the data related to those 10 grids. If you have no access to folder Z that contains 30 project grids, you will not see any data from those grids.
To create a new dashboard you simply click the + button and a new blank dashboard will be created. New dashboards are only accessible to you and your account architects, you can then choose to give access to other members of your account as required. Find more information on dashboard access control here.